FIG. 24 illustrates an example of a conventional communication module incorporating two tuners. In FIG. 24, the communication module is equipped with one circuit board 1 with a substantially quadrangle shape, and on the first main surface of which first tuner unit 2a, and second tuner unit 2b and demodulating unit 3 are implemented respectively.
One first vertical side 1a of circuit board 1 is provided thereon with first terminal 5a to which first antenna 4a is connected and second terminal 5b to which second antenna 4b is connected, respectively.
First tuner unit 2a includes first filter 6a connected to first terminal 5a, for processing high-frequency signals; first amplifier 7a connected to the output terminal of first filter 6a, for amplifying high-frequency signals; and first mixer 9a having a first input terminal to which the output terminal of first amplifier 7a is connected and a second input terminal to which the output terminal of local oscillator 8a is connected. First tuner unit 2a includes intermediate-frequency filter 10a connected to the output terminal of first mixer 9a; and intermediate-frequency amplifier 11a connected to the output terminal of intermediate-frequency filter 10a. The output terminal of intermediate-frequency amplifier 11a is connected to first terminal 12a of electronic switch 12.
Second tuner unit 2b includes second filter 6b connected to second terminal 5b, for processing high-frequency signals; second amplifier 7b connected to the output terminal of second filter 6b, for amplifying high-frequency signals; and second mixer 9b to which first input terminal the output terminal of second amplifier 7b is connected and to which second input terminal the output terminal of local oscillator 8b is connected. Second tuner unit 2b includes intermediate-frequency filter 10b connected to the output terminal of second mixer 9b; and intermediate-frequency amplifier 11b connected to the output terminal of intermediate-frequency filter 10b. The output terminal of intermediate-frequency amplifier 11b is connected to second terminal 12b of electronic switch 12.
Common terminal 12c of electronic switch 12 is connected to output terminal 13 through demodulating unit 3. In the communication module formed as described above, first terminal 5a and second terminal 5b are supplied with a first signal and a second signal for television broadcasting respectively, for example. In order to receive the first and second signals with high fidelity, demodulating unit 3 compares output from first tuner unit 2a with that from second tuner unit 2b and chooses tuner unit 2a or tuner unit 2b, whichever is more sensitive, with electronic switch 12. This always enables reception with high sensitivity.
As prior art documents related to this patent application, Japanese Patent Unexamined Publication No. 2003-18123 is known, for example.
In such a conventional communication module, first tuner unit 2a and second tuner unit 2b are implemented on common circuit board 1. This causes high-frequency separation and isolation between first tuner unit 2a and second tuner unit 2b to be inadequate. In particular, the following problem occurs. That is, a feeble reception signal from first terminal 5a is amplified to increase its signal energy during the period after the first signal is amplified by first amplifier 7a until the signal is converted to an intermediate-frequency signal (low-frequency signal) by first mixer 9a. Accordingly, the first signal is mixed into second tuner unit 5b to interfere with the second signal.
Meanwhile, a feeble reception signal from second mixer 9b is amplified to increase the signal energy during the period after the second signal is amplified by second amplifier 7b until the signal is converted to an intermediate-frequency signal (low-frequency signal) by second mixer 9b. Accordingly, the second signal is undesirably mixed into first tuner unit 5a to interfere with favorable reception.